Technical Field
The present invention relates to a power conversion apparatus that includes a semiconductor module including semiconductor elements, a cooler cooling the semiconductor module, and a controller controlling switching operations of the semiconductor elements.
Related Art
As a power conversion apparatus converting between DC power and AC power, an apparatus is known that includes a semiconductor module, which includes semiconductor elements such as IGBTs, and a controller connected to the semiconductor module (refer to JP-A-2008-178200). The power conversion apparatus uses the controller to make the semiconductor elements perform switching operations, thereby converting DC power to AC power.
Since the power conversion causes heat generation from the semiconductor elements, the power conversion apparatus cools the semiconductor elements by using a cooler. In the cooler, a flow path is formed through which a coolant flows. Heat exchange between the coolant and the semiconductor elements cools the semiconductor elements.
In addition, the power conversion apparatus of JP-A-2008-178200 is provided with a temperature sensor in the semiconductor module to measure a temperature (hereinafter, also referred to as an element temperature) of the semiconductor element. The controller controls turn-off speed of the semiconductor element based on the measured value of the element temperature. That is, for example, if the element temperature becomes a predetermined threshold value or more, the controller increases the turn-off speed. If the element temperature becomes less than the predetermined threshold value, the controller decreases the turn-off speed. Thereby, the element temperature is prevented from excessively rising, and the semiconductor element is protected from a turn-off surge.
That is, increasing the turn-off speed decreases the amount of heat generation (loss), and decreasing the turn-off speed increases the amount of heat generation. Hence, since increasing the turn-off speed when the element temperature has become high can decrease the amount of heat generation, the element temperature can be prevented from excessively rising. Accordingly, the element temperature can be prevented from exceeding a predetermined upper limit value.
In addition, as described later, increasing the turn-off speed when the element temperature is high and decreasing the turn-off speed when the element temperature is low can also protect the semiconductor element from a turn-off surge.
However, the power conversion apparatus is difficult to reduce the manufacturing cost thereof while preventing the element temperature from exceeding the upper limit value. That is, when the power conversion apparatus is operating, the temperature of the coolant (hereinafter, also referred to as a coolant temperature) may be relatively high, and the element temperature may be lower than the threshold value. In this case, since the element temperature has not exceeded the threshold value, the controller decreases the turn-off speed. Hence, the amount of heat generation of the semiconductor element becomes high. Accordingly, although the coolant temperature is high, and it is difficult to cool the semiconductor element, the amount of heat generation becomes large. Thereby, the element temperature may sharply rise to exceed the upper limit value.
Even in the above case, to prevent the element temperature from greatly exceeding the upper limit value, it is required to enlarge the area of the semiconductor element to decrease the amount of heat generation per unit area so as to restrain the amount of temperature rise of the semiconductor element. Accordingly, the manufacturing cost of the semiconductor element easily increases.